The purpose of this work was to study the chemical and structural properties of unique unesterified cholesterol-rich lipid particles isolated from the atherosclerotic lesions of human and rabbit aortas. These lipid particles were purified from aortic extracts by microfiltration, gel filtration and density gradient ultrcentrifugation. The purified lipid particles had a hydrated density between 1.02 g/ml and 1.08 g/ml with a peak at 1.036 g/ml. The lipid particles had a high molar ratio of unesterified cholesterol to phospholipid (2.4:1 in rabbit, 2.6:1 in human) and a high percentage of their cholesterol in an unesterified form (82% in rabbit, 76% in human). Particles from both human and rabbit aortas were similar in size and shape. They had a modal diameter of 1000-1200 Angstrom unit (size range between 700-3000 Angstrom unit) and appeared singly or in aggregates with uni- and multi-lamellar structures sometimes containing a non-aqueous core. Their fracture faces were smooth and devoid of intramembranous particles, indicating the membranes of these particles were not cellular membranes. Upon incubation with filipin, the particles showed typical filipin- sterol complexes, demonstrating the presence of unesterified cholesterol. Glycosaminoglycan (GAG) were examine as a cause of the tendency of the lipid particles to aggregate. Incubation of lipid particles with chondroitinase ABC successfully digested the GAG content of the particles but did not appear to change the degree of particle aggregation. This suggests that GAG may not be a major factor mediating aggregation of this lipid particles. A substantial amount of free fatty acids was found associated with the lipid particles (8% of total lipid, molar basis). The presence of such amounts of free fatty acids is consistent with the possibility that these particles might be a degradation product of plasma lipoproteins.